1. Field of the Invention
Embodiments of the invention relate to bend stiffeners for flexible members. More particularly, embodiments of the invention relate to bend stiffeners for use with cables for interconnecting one or more seismic stations placed at subsea locations with suitable control and monitoring equipment.
2. Description of the Related Art
A bend stiffener is typically used at cable terminations at cable splice units, regenerator units, or the like in subsea cable applications where it is desirable to improve the load capacity of the cable and cable termination. A cable termination is typically installed with the cable and attached units from a cable ship or other installation vessel.
Lateral movements of a subsea cable may occur due to environmental loading, underwater currents or handling during installation and removal of the cable. In particular, the cable proximate a splice unit, regenerator or sensor unit may be subject to large bending forces in the above situations due to a combination of angular misalignment and tension. If suitable protective means are not used, the bending forces may exceed the bending capacity of the cable causing undesirable tear, fracture or even full breakage. It is a common technique to use bend stiffeners to improve the bending capacity of cables at such cable terminations, thus increasing the bend radius of the bending curve of the cable for a given load on the cable.
For the cable terminations three typical load situations occur. Firstly, the small loads (2-20 times the weight of the unit) and large angular displacements (up to 90 degrees) occurring when the unit is raised vertically from horizontal storage. Secondly, in connection with the onboard handling and movement over a running wheel of the ship (typical diameter 3-6 m) with strain from the cable etc. (determined by cable weight, ocean depth, etc.), the typical load is 1.2-1.5 times the weight of the free cable in water. Over the running wheel a typical angular displacement from the axis of 25-40 degrees may occur. The last situation happens when lifting a unit from the ocean bottom. If the unit has been buried (for protection purposes or due to movement of material on the ocean bottom) for a long time (10-30 years) large forces may be required to lift the unit. The cable is typically drawn almost vertically upwards (at up to 90 degrees to the direction of the cable at the ocean bottom), and the cable and unit is stuck in nearly “coagulated” mass. Fortunately, experience indicates that the loosening process provides a shaping of the cable which somewhat limits the bending of the cable. Normally, it is not required that the cable need to be undamaged when it is taken out of the seafloor.
Ocean Bottom Seismic (OBS) systems, which may be at least partly buried on the ocean bottom, include hydrophones intended to measure pressure waves in the ocean at or near the ocean bottom. The hydrophone is a part of a measuring station which may also contain geophones or accelerometers, and is normally protected by a stiff external housing. On the ocean floor, the station may be left as a unit enclosed by a mass of loose sediments from a flushing operation (or ploughing). Sand may also enter into the open spaces of the station and partly fill the holes which have been provided for allowing entry of pressure waves.
Such sediment mass may become fixed or hardened during time when the particulate matter (sand, silt, clay, etc.) packs together. Dependent on the bottom conditions, this mass may become relatively rigid and have a large resistance to water flowing through it. If the compressibility of the volume that the station represents becomes high (low bulk modulus) in relation to the surrounding masses, the pressure build-up may require some time to occur. The hydrophone thus may measure a lower response for signals of high frequencies than for static pressures.
OBS systems include a cable and stations having metallic parts necessary for, e.g., mounting and connecting the different parts to each other. These may be subject to corrosion when placed on the ocean bottom. A common method of avoiding such problems is to provide cathodic protection. Sink anodes are attached to the unit and these serve to prevent the corrosion of metallic parts of the station/cable. Cathodic protection is a galvanic process which involves the dissociation of water. This process may in some cases cause the formation of gas. If the gas is collected in the station (or in the cavity this represents when buried), it may in effect result in a very high compressibility (low bulk module). Measurements of pressure waves may thus be strongly affected (see above paragraph).
There exists a need for an improved bend stiffener suitable for use with ocean bottom cables intersected by subsea units, such as seismic stations, splice units, and regenerator units, which improves the bending curve and/or load distribution along the bend stiffener and the associated end of the cable when such a cable with attached units are being installed and removed, typically to and from locations at large sea depths. A further need exists to reduce or eliminate the problems related to gas formation, especially from cathodic protection when the cable with the units is placed on the sea bottom.